1. Field of the Invention
This invention is in the field of electrophotography and more particularly relates to an optical toner sensing and toner replenishment system.
2. Description of the Prior Art
In the customary electrophotographic processes, a conductive backing having a photoconductive insulating layer thereon is electrostatically imaged by first uniformly charging its surface, and subsequently exposing the charged surface to a pattern of activating electromagnetic radiation, such as light. The radiation pattern selectively dissipates electrostatic charges in the illuminated area on the photoconductive surface, which results in the formation of a latent electrostatic image in non-illuminated areas. This latent electrostatic image can be developed to form a visible image by depositing developer materials thereon by a variety of development techniques, the most common of which is cascade development. The developed toner image is then transferred and fused to another substrate such as plain paper.
Typically, cascade development apparatus includes a housing including within it a bucket elevator system formed on an endless belt having buckets attached thereto. Electrostatic developer, consisting of carriers and toner particles, is lifted from a reservoir section in the buckets to a point at the upper portion of a photoconductive drum and is then cascaded over the drum surface by means of a feed guide. As developer cascades over the drum, toner particles separate from the carrier beads and deposit on the drum surface in accordance with the latent electrostatic image thereon, thus forming a visible toner image. Spent developer is guided back into the reservoir and is eventually lifted back to the upper portion of the drum by the elevator system. As development occurs, the concentration of toner particles within the system is, of course, depleted. Beyond the reduction in toner concentration, the image-development state is also affected by the charge-to-mass ratio existing on toner particles. This is affected, of course, by factors such as humidity and machine useage.
There are a variety of optical toner sensing systems described in the patent literature. Several of these operate by transmitting light through a developer layer. Typical of transmission sensing systems are those described in U.S. Pat. Nos. 3,430,606; 3,526,338; 3,791,744; 3,778,146. Such systems have a significant drawback, however, in that the optical path can become contaminated with toner particles thereby reducing the apparent transmission and interfering with accurate readout.
One patented system intended to overcome this problem is that described in U.S. Pat. No. 3,778,146. In this system, an area of the photoconductive drum located outside the imaging area is fabricated from a transparent material so that toner particles can be deposited thereon. An optical detector and light source are then used, but this system suffers because of the complicated drum configuration and the necessity to place either the sensor or light source within the drum.
Other optical toner sensing systems described in the patent literature involve the differential reflection from a developer mass and the use of a calibrated reflecting source. See, for example, U.S. Pat. No. 3,756,192. This system entails the use of a segment of the developer which is bypassed and intermittently sampled.
U.S. Pat. No. 3,610,205 discloses yet another system wherein the developer mix is passed by a window and light is passed through the window and reflected off this mix and subsequently sensed.
None of these systems, however, actually test the image-development state of developer as it exists at any finite moment in the system.